Change `horizon` in `step_growth_rate()` and `arx_class_args_list()`

We sometimes use horizon interchangeably with ahead and so perhaps we should change the name of it in `step_growth_rate()` and `arx_class_args_list()` (and anywhere else it gets used in such a way in this package). Since it is passed to the `h` argument of `epiprocess::growth_rate()`, then maybe the easiest thing is to just refer to it as `h`?